Does iMessage to iMessage (both iphones) use texting?

My friend has and iPhone but doesn't have unlimited texting, if I have and iphone and we're messaging does it use her text? Or does iMessage not go through text but it separate

IMessage uses small amounts of data to send the message rather than dipping into your text message limit.

Also if you have AT&T then you can download the my AT&T app to keep track of the amour of messages you have left as well as your data usage.

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